1. Field of the Invention
The present invention relates to an array antenna which is used in a frequency band such as a millimeter band, a microwave band, and the like, and uses a planar resonator, and more particularly, to a planar array antenna, which can easily perform transmission and reception of an orthogonal polarization and a circular polarization, and can easily perform transmission and reception at plural frequency bands.
2. Description of the Background Arts
In general, a planar antenna can be easily fabricated and processed, and made compact and light in weight. Hence, It finds wide use in the field of radio communications, satellite broadcasts, and the like. Accompanied by development and diversification of the radio communications in recent years, the planar antenna has been also expected to have high performance and sophisticated features. In U.S. Pat. No. 6,753,817, the present inventors have proposed a multi-element planar antenna, which can share polarization components, and can use a circular polarization.
FIGS. 1A and 1B illustrate a conventional multi-element planar antenna. This planar antenna comprises four antenna elements 2a to 2d formed on substrate 1 made from dielectric materials and the like, and a feeding system for these antenna elements. Each of antenna elements 2a to 2d is configured as a planar resonator of a microstrip line type, and is specifically comprised of square resonance conductor 3 provided on one principal surface of substrate 1, and ground conductor 4 formed on an almost entire surface of the other principal surface of substrate 1. The centers of antenna elements 2a to 2d are positioned at each apex of a geometrical square, in the example shown here, a regular square.
The feeding system comprises first to fourth microstrip lines 5, 6, 7 and 8 provided on one principal surface of substrate 1, and first and second slot lines 9 and 10 provided on the other principal surface. First and second slot lines 9 and 10 are formed as slot lines having the same length and being short-circuited at both ends, and extend in mutually orthogonal directions, and at the same time, mutually intersect at the median point thereof. This intersection is equal to the center of the geometrical square. In the figure, first slot line 9 extends in the vertical direction, and second slot line 10 extends in the horizontal direction. That is, slot lines 9 and 10 are formed in the shape of a cross as a whole. As will be described later, four corners formed at a position where slot lines 9 and 10 intersect each other in ground conductor 4 becomes feeding positions for this planar antenna.
Any of microstrip lines 5 to 8 is the same in length, and as a whole, is formed along the side of the regular square. Antenna element 2a at the left above in the figure is connected to the upper end of microstrip line 7 and the left end of microstrip line 5, and is fed at two points from these microstrip lines 7 and 5. Similarly, antenna element 2b at the right above in the figure is connected to the upper end of microstrip line 8 and the right end of microstrip line 5, and antenna element 2d at the left below in the figure is connected to the left end of microstrip line 6 and the lower end of microstrip 7, and antenna element 2c at the right below in the figure is connected to the right end of microstrip line 6 and to the lower end of microstrip line 8. These microstrip lines 5 to 8 are orthogonal to these slot lines 9 and 10 so as to traverse them, respectively, at equally distant positions in the vertical and horizontal directions from the intersection of slot lines 9 and 10, and are electromagnetically coupled to these slot lines. With the guide wavelength corresponding to the antenna design frequency of this planar antenna taken as λ, the top end of each of slot lines 9 and 10 becomes a short-circuit end edge, but it is preferable that the top end is allowed to extend approximately λ/4 in length from the traversing point with the microstrip line. If configured in this manner, in the antenna design frequency, the top end of each of slot lines 9 and 10 electrically functions as an open end seen from the traversing point with the microstrip lines, and in this manner, propagation efficiency from the feeding point to the antenna element through slot lines and microstrip lines is enhanced.
In this planar antenna, each of antenna elements 2a to 2d has a degenerate mode in horizontal and vertical orthogonal directions. The electrical length from the intersection of first and second slot lines 9 and 10 to each of antenna elements 2a to 2d through microstrip lines 5 to 8 is the same.
As described above, in this planar antenna, four corners in ground conductor 4 formed at the position where first and second slot lines 9 and 10 intersect each other become feeding positions fed with high frequency signals. Hence, for the sake of simplicity, these four corners will be referred to as a, b, c, and d clockwise from the left above in the figure.
First, among the four corners in the feeding position, corners a and b located at the upper side of second slot line 10 are made a pair, and corners c and d located at the lower side of second slot line 10 are made another pair, and between corners a and b, and corners c and d, high frequency signals are fed. As a result, in second slot line 10 extending in the horizontal direction, a high frequency component is excited in the electric field direction shown by an arrow. This high frequency component is propagated to both ends of second slot line 10, and electromagnetically couples with the microstrip lines at each median point of third and fourth microstrip lines 7 and 8. Since the conversion from a slot line to a microstrip line is a reverse-phase series branch, the high frequency propagated to microstrip lines 7 and 8 is reversed in electrical field, respectively in the vertical direction in the figure, and is propagated in a reverse phase. While, seen from second slot line 10, antenna elements 2a and 2b at the upper side in the figure and antenna elements 2c and 2d at the lower side in figure are fed with high frequency signals in reverse phase, since the feeding points of the antenna elements are in mirror symmetry, each antenna is excited in-phase. In this case, since the feeding in the vertical direction is made to each of antenna elements 2a to 2d, a vertical polarization is radiated.
Among four corners a, b, c, and d in the intersection of slot lines 9 and 10, if corners a and d located at the left side of first slot line 9 are made a pair, and corners b and c located at the right side of second slot line 9 are also made a pair, and the feeding is made between corners a and d and corners b and c, a high frequency component is excited in first slot line 9 extending in the vertical direction. This high frequency component is propagated from the median points of first and second microstrip lines 5 and 6 to both end sides of these microstrip lines by electromagnetic coupling. At this time, in each of microstrip lines, when seen from the median points thereof, the electric field is reversed, and the high frequency component is distributed in reverse phase in the horizontal direction. As a result, similarly to the aforementioned case, in each of antenna elements 2a to 2d, high frequency is fed in-phase in the horizontal direction, and is radiated as a horizontal polarization from these antenna elements. Here, since the shape of the antenna element is made regular square, the antenna frequency by means of the vertical polarization and the antenna frequency by means of the horizontal polarization correspond to each other.
In the planar array antenna shown in FIGS. 1A and 1B, a functional device such as an integrated circuit (IC) and the like is connected to the vicinity of the intersection of first and second slot lines 9 and 10, and if the space between the corners located in the vertical direction or the space between the corners located in the horizontal direction are selected and fed, in other words, if the space between corners a and b, and corners c and d, or the space between corners a and d, and other corners are selected and fed, the vertical polarization or the horizontal polarization can be switchably transmitted by a single array antenna.
Further, according to this planar array antenna, through the feeding between the corners located in a diagonal direction, that is, through the feeding either between corners a and c or between corners b and d, a linear polarization can be transmitted in a direction to tilt 45 degrees in the upper or lower direction, respectively, from the horizontal direction in the figure. Further, through the provision of a delay circuit, it is possible to transmit the circular polarization, and through the change of the shape of each antenna element, a planar array antenna sharing plural antenna frequencies can be configured. It is apparent that, in consideration of reversibility in the antenna, receiving operation is possible also by the reverse action of transmitting operation.
However, in the planar array antenna thus configured, a functional device for feeding is connected to the intersection between first and second slot lines, and for example, a feeding cable is connected so as to extend in the vertical direction to the substrate surface. Hence, the antenna including the feeding cable becomes three-dimensional, and surfaceness or compactness of the antenna is prevented.
Hence, in the above described U.S. Pat. No. 6,753,817, the present inventors have proposed a structure in which a dielectric substrate is also disposed on ground conductor 4 in the planar array antenna shown in FIGS. 1A and 1B, and make the antenna into a multi-layer substrate structure, and on the surface of that dielectric substrate, a feeding microstrip line is disposed. Here, ground conductor 4, which becomes an intermediate layer conductor of the multi-layer substrate, is formed with first and second slot lines, and the feeding microstrip line on the dielectric substrate extends till a position corresponding to the intersection of first and second slot lines, and feed these first and second slot lines. Here, through extending the feeding microstrip line in the diagonal direction in the intersection, for example, in the direction to connect corner b and corner d, it is possible to transmit the linear polarization in a direction to tilt 45 degrees to the right from the vertical direction. Through the change of the arrangement of the feeding microstrip lines, the vertical polarization and the horizontal polarization can be transmitted and received.
However, to make such a planar array antenna shareable with the vertical polarization and the horizontal polarization, the feeding microstrip lines must intersect each other on the dielectric substrate. Further, while it is conceivable to provide feeding microstrip lines for first and second slot lines on the principal surface side on which the antenna elements are provided, such feeding microstrip lines intersect any of the first to fourth microstrip lines 5 to 8 which connect between antenna elements 2a to 2d. 
Eventually, in the case of the planar array antenna shown in FIGS. 1A and 1B, it is difficult to constitute a feeding system by the microstrip lines, while sharing the vertical polarization and the horizontal polarization, and therefore, it is difficult to construct the antenna including the feeding system in a planar manner.